1. Field of the Invention
This invention relates to a flat type display device for display of a picture image or a projected image, and more particularly to a flat type display device formed into a thin shape and adapted to carry out the display with high luminescence and definition.
2. Description of the Prior Art
In general, a display device employing a cathode ray tube (CRT) has been conventionally used for displaying a picture image or a projected image. Such a CRT-type display device is adapted to scan, on a phosphor-deposited screen, one or more electron beams generated from an electron gun and impinge electrons on a phosphor deposited screen at a high velocity. Accordingly, the conventional CRT-type display device exhibits the advantages of using high-velocity electron excited phosphor, carrying out colored display and accomplishing the display with high luminescence and definition.
However, the CRT-type display device is disadvantageous in that it is very hard to reduce a thickness of the CRT-type display device as well as its weight, because the electron gun must be arranged behind a display plane and it is required to increase its depth in order to scan electron beams between both ends of the screen.
Also, thin-type display devices have been developed and partially put to practical use which are adapted to display a picture image or a projected image in place of the CRT-type display device. Such thin-type display devices include a fluorescent display device, a liquid crystal display device, an electroluminescence display device, a plasma display device and the like.
Unfortunately, the thin-type display devices have the following important disadvantages. The fluorescent display device carries out colored display, however, the colored display lacks definition. Also, it fails to exhibit luminescence and life characteristics suitable for display of a picture image and a projected image, as well as cannot be large-sized to a degree sufficient to carry out the display. The electroluminescence display device and plasma display device each fail to provide colored display and exhibit satisfactory luminescence and life characteristics. The liquid crystal display device is of the non-emission type, so that it may not provide sufficient luminescence even when it is used in combination with a back light device. Also, the liquid crystal display device has a small angle of visibility, resulting in its display quality being substantially deteriorated. Thus, the liquid crystal display device likewise is insufficient to display a picture image and a projected image.
Further display devices are also proposed as disclosed in Japanese Patent Application Laying-Open Publication No. 48345/1983 and Japanese Patent Application Laying-Open Publication No. 171440/1984. The display devices each are adapted to use high-velocity electron excited phosphor.
More particularly, the display device taught in Japanese Patent Application Laying-Open Publication No. 48345/1983 includes a phosphor-deposited screen, an electron source including a plurality of filamentary cathodes arranged opposite to the screen and stretched in a horizontal direction with respect to the screen, and a control electrode group adapted to selectively draw out an electron beam from the cathodes and deflect it in a vertical or horizontal direction with respect to the screen. Luminous display is obtained by focusing electrons emitted from the cathodes into an electron beam and carrying out selection and deflection of the electron beam by means of the control electrode group, to thereby selectively impinge the electron beam on the screen.
The display device taught in Japanese Patent Application Laying-Open Publication No. 171440/1984 includes filamentary cathodes adapted to planely emit electrons, a screen having phosphor layers arranged in a stripe-like manner in a direction perpendicular to the filamentary cathodes, control electrodes for focusing the electrons into a beam-like shape and selectively forming the so-focused electrons into electron beams corresponding to the stripe-like phosphor layers, a deflection coil for deflecting the electron beams along the stripe-like phosphor layers, and a back electrode.
In the former display device or the device taught in Japanese Patent Application Laying-Open Publication No. 48345/1983, the filamentary cathodes are positioned opposite to the screen and electrons are impinged on the phosphor layers at a high velocity, resulting in phosphor deposited on an inner surface of the screen being decomposed. The so-decomposed phosphor then adheres to the filamentary cathodes to deteriorate electron emission capability of the filamentary cathodes, to thereby shorten a life of the device. Alternatively, oxide formed on a surface of each of the filamentary cathodes is decomposed and then adheres to the phosphor deposited on the screen to deteriorate emission efficiency of the phosphor, resulting the display device being short-lived.
Also, excessive deflection lines in vertical and horizontal directions render control of the electrons difficult and cause the diameter of the electron beam to be increased which leads to bleeding. Thus, it is not desirable to increase the number of deflection lines in a direction perpendicular to each filamentary cathode. Accordingly, it is required to increase the number of filamentary cathodes. However, this not only leads to an increase in power consumption but requires to carry out assembling of the display device with high accuracy because even slight misregistration of the filamentary cathodes with respect to other electrodes adversely affects display by the device, resulting in the assembling being highly troublesome. Further, in the display device, electron beams are formed directly from the filamentary cathodes, so that vibration of the filamentary cathodes leads to a variation of the electron beams. Unfortunately, this causes color shift and/or bleeding to occur in the display. Thus, the display device is not suitable for display on a large image plane in which long filamentary cathodes are required.
In the latter display device or the device taught in Japanese Patent Application Laying-Open Publication No. 171440/1984, the electron beams are deflected over a whole length of the stripe-like phosphor layers, resulting in the amount of deflection of the electron beams being increased. This renders uniform focusing of the electron beams over the whole length highly difficult. Also, deflection of the electron beams at a region adjacent to the display plane is carried out by a combination of the above-described back electrode and a mesh-like electrode or a phosphor electrode (final beam acceleration electrode). Accordingly there occurs a difference in angle of electron beams impinging on the phosphor between a portion of the screen far away from the electron source and a portion of the screen near the electron source. This causes electron beams impinging on the phosphor to be elongated particularly at the portion of the screen far away from the electron source although the electron beams are generally elongated in a direction perpendicular to the cathodes, resulting in a failure in a display with high definition.